IgG antibodies are excellent acceptors of nascent C3b and form covalent complexes with C3b in immune complexes and on bacterial surfaces. In addition, complement activation in serum leads to the generation of non-specific C3b-IgG complexes from bystander monomeric IgG. We have reported that C3b-IgG complexes can be readily formed in vitro and have delineated a purification schema which allows their preparation in good yield from purified C3 and IgG. C3b in such hetero-dimeric complexes is bound primarily to IgG heavy chains by an ester linkage. Such C3b shows a retarded rate of inactivation by factors H and I relative to free, uncomplexed C3b. This retardation of inactivation is primarily due to reduced affinity of C3b complexed to IgG for factor H. In the presence of adequate levels of factor H, the action of factor I on C3b-IgG heterodimers is not impeded by the IgG molecule. Substitution of an immunologically inert glycoprotein molecule, ceruloplasmin, for IgG results in hetero-dimeric complexes of C3b-ceruloplasmin. C3b in such complexes displays no retardation of inactivation by factor H and I, suggesting that inhibition of such inactivation may be a peculiar property of IgG. C3b-IgG complexes are much more efficient than C3b on a molecule for molecule basic in promoting alternative pathway complement activation. These data may explain the role of IgG in enhancing alternative pathway activity and also may have important bearing on our understanding of immune complex metabolism.